Apparatus for developing latent electrostatic images



Feb. 20, 1968 G. NAUMANN 3,369,523

APPARATUS FOR DEVELOPING LATENT ELECTROSTATIC IMAGES Filed Aug. 8, 1966 iNVENTOR GERHARD' NAUMANN I BY Z M ATTORNE United States Patent flfice ABSTRACT OF THE DISCLOSURE This invention relates to an apparatus, for the development of latent electrostatic images with a dispersion developer, which comprises an application roller, wiper means in engagement with the roller and defining a reservoir between itself and the periphery of the roller, means for maintaining a supply of developer liquid in the reservoir, and means for passing an image carrier past the application roller on a side of the latter opposite the wiper means whereby developer is transferred from the roller to the image carrier.

Dispersion developers, as used in electrophotography, are fluid developers and comprise a dispersion of solid toner particles in a fluid medium having a high specific resistance and a low dielectric constant. When a developer of this type is brought into contact with a surface electrostatically charged in the form of an image, the toner particles are attracted to the surface in accordance with the charge pattern while the dielectric fluid flows off; a visible picture is thereby produced. Positive or negative copies of the image to be reproduced can be obtained depending upon the sign of the charge of the latent electrostatic image to be developed and the sign of the charge on the dispersed particles.

If the latent image charge and the charge of the dispersed particles are the same, a negative image is obtained, while if the charges are opposite a positive copy is produced; the latter case is the usual one. The known devices for developing latent electrostatic images with dis persion developers are suited to this method. Conversely, the present invention preferably relates to an apparatus for inverse development of latent electrostatic images with dispersion developers.

In older known devices for developing latent electrostatic ima-ges with dispersion developers it is the normal practice to bring the electrophotographic material into contact with the developer, and thus develop it, by immersing it or drawing it through a container containing developing fluid.

It is also a known practice to develop the electrophotographic material by bringing it into contact with a thin film of developer fluid on the surface of a rotating roller. Apparatus of this type has been developed from older known devices used in the fields of diazotype and silver photography.

It is a disadvantage of the older known devices with rotating rollers that a so-called impression is generally produced by the rotation of the roller, i.e., the toner particles still orientated in picture form from the previous I developing process are printed again onto the newly introduced electrophotographic material. It therefore previously has been proposed that one or more scrapers pressing against the surface of the application roller should be used to create a fresh surface and thus prevent imprinting. However, as the scrapers easily drag longitudinal stripes which again can mar the developed picture, it was suggested that the developer fluid should be sprayed onto the roller from a nozzle so that it would be clear.

3,369,523 Patented Feb. 20, 1968 It is a common feature of the known devices for dispersion development of latent electrostatic images that they are designed in such a way that the developer comes in contact with the surface of the layer in as turbulent a stream as possible. This means that the greatest possible number of dispersed particles are available for deposition per unit time so that no impoverishment of particles can occur due to ditfusion. The turbulence of the developer fluid is achieved by stirring or spraying and also by relative motion between the material to be developed and the application roller.

The known devices are all suitable for positive development and some give outstanding copies. However, it is not possible to use them to produce reverse copies of satisfactory quality. In the case of reversal development, the non-charged parts of the image must be made visible, i.e., the dispersed toner particles must be deposited on these parts. This can, of course, be achieved only with the help of the electrostatic field generated by the charged parts of the image, This must extend as far as possible into the uncharged portions in order to obtain a suflicient deposit of toner particles. The field distribution during the developing process obviously also is influenced by the components of the developing device. Theoretical observations as to how the developing apparatus should be made in order to obtain optimum reversal development are scarcely possible, however, in view of the complexity of the question, but it is a fact that the known developing devices do not offer optimum conditions for reversal development. Thus in developing apparatus with roller application in which the coating on the surface of the image carrier is brought into direct contact with the material of the roller, very blurred and poor contrast copies are obtained.

The present invention provides apparatus for the dispersion development of latent electrostatic images which is especially suitable for reversal development. This means the apparatus must be designed in such a way that the rear side of the image carrier does not come in contact with the dispersion developer and the front side should not be in direct contact with the roller material during developing.

Wet development of latent electrostatic images is based on similar considerations. In this case, it has already been suggested that the development should be performed by means of an application roller while the carrier is supported at the rear by a strip with two linear rests. However, a corresponding device suitable for reversal development of latent electrostatic images does not entirely fulfill the above requirements in the case of reversal development with a dispersion developer since the contact zone between the surface of the image carrier and the dispersion developer is too small and also the contact is too narrow.

The apparatus of the invention comprises an application roller, a wiper member arranged to engage the application roller to define between itself and the uppermost edge of the roller a trough-shaped reservoir, means for maintaining a supply of developer liquid in the reservoir, and means for transporting an image carrier downwardly past the lower region of the application roller on the side opposite the wiper member. The arrangement is such that, during operation of the apparatus, the image carrier can be held lightly against th application roller and the latter can be driven in the same direction and at the same or approximately the same speed of movement as the image carrier, developer liquid being carried by the roller upwardly from the reservoir and over the upper edge of the roller to pass downwardly onto the image carrier, while a film of developer liquid is maintained between the surface of the application roller and the image carrier.

The invention will be further illustrated by reference to the accompanying drawings, in which:

FIGURE 1 is a sectional elevation of one form of apparatus according to the invention, and

FIGURE 2 is a similar view showing an auxiliary rinsing device.

Referring to FIGURE 1, the image carrier is transported, by a pair of feed rollers 6, into which it is introduced with the image side uppermost, past the developer roller 1. It is lightly supported by a guide element 8 so that the sheet is to be regarded as freely guided. The now developed but still damp sheet slides over an element 10, in the form of a trough and preferably consisting of wires parallel to one another in the longitudinal direction, into a pair of take-up rollers 11 which also serve as squeezing rollers. The upper roller of the pair of squeezing rollers 11 is made of polished metal. The now developed and largely dried sheet is advanced by this pair of rollers onto the receiving table 12.

A film of developer fluid is applied to the application roller 1 by means of a wiper member 2 and a nozzle 3. The wiper 2 is mounted in such a Way that a troughshaped reservoir is formed between it and the roller in which the fluid builds up in a stationary supply. The wiper can be made of rubber or other rubber-like synthetic materials, felt, cloth, or similar materials resistant to wear. The wiper is preferably rectangular and of a height of -20 mm. The nozzle 3 sprays the dispersion developer into the gap 4 already mentioned. The developer is fed to the nozzle through a pipe 5, from a storage container, not shown, by a pump, also not shown.

The nozzle can be a slot nozzle or a tube with a number of small holes. The pressure, which is regulated by the pumping rate, should be set so that the developer fluid does not foam in the reservoir 4. A small pool of developer fluid is formed in the reservoir 4, part of it being continually carried away by the application roller. The part of the fluid film which is not transmitted to the image carrier is held back by the wiper 2 and flows into the container 9 whence it passes to the storage container already mentioned but not shown. However, the greater part of the fluid sprayed into the reservoir 4 flows to the two ends thereof and over the end surfaces of the application roller to be caught also in the container 9.

In this way, a very uniform and relatively thick film of the dispersion developer is formed on the application roller 1. Due to the force of gravity, the thickness of the layer is greater towards the bottom of the application roller. On the surface of the roller in the bottom, right hand quadrant in FIGURE 1 the film of developer is undisturbed by developer fiowing back from the reservoir 4. The developing process takes place in this region of the circumference since the greatest quantity of developer fluid is available there and, at the same time, there is least danger of the surface of the application roller coming into direct contact with the carrier. Thus, an unbroken film of developer fluid is maintained between the freely advancing image carrier and the metal mass of the application roller which influences the electrostatic field.

The application roller can be made of any metal. The best are brass, chromed brass or high-grade steel, as these materials easily can be given a high quality surface finish. The diameter of the application roller can be between 20 and 100 mm. Smaller or larger diameters are also possible under certain conditions. The width of the roller can be of any desired magnitude subject to structural requirements and is limited only by the tendency of the roller to bend under its own weight. However, this tendency is insignificant in rollers of the widths used in practice.

The peripheral speed of the application roller should be at least 2 m. per minute and should not exceed 25 m. per minute. At lesser and greater speeds the quality of the copies is considerably reduced.

The purpose of the guide element 8 is to support the image carrier from the rear during developing. The form of the guide element 8 is not critical. All that the element has to do is to ensure that the copy material arriving approximately tangentially to the application roller is guided in a slight curve without being pressed hard against the roller. Thus, for example, a curved sheet of metal with the same curvature as the application roller over at least part of it can be used as the guide element. The form of guide element shown in FIGURE 1 is particularly advantageous and therefore is preferred. As shown, a sheet of metal bent in two places is mounted in such a way that the edges 8a and 812 form two guide rails parallel to the roller. The distance of the edges from the roller can be between 2 and 20 mm. It is not necessary for both edges to be at the same distance from the roller. It is even possible to use only a strip of metal, the front edge of which is parallel to the axis of the application roller.

If particularly high quality reproduction is to be achieved, it is recommended that the image be rinsed with the pure dispersion medium after the developer has been applied and before the excess developer is squeezed off. Provided this rinsing is carried out in the dark, developer particles adhering to the background will be washed off while the developer will continue to adhere to the required parts of the image. Fluid hydrocarbons or fluid halogenated hydrocarbons generally are used as the dispersion medium. The rinsing can be carried out simply by immersing the just-developed image in the pure dispersion medium. However, this method is not particularly successful and it is preferable to carry out rinsing with a moving fluid. A device such as that shown in FIGURE 2 is particularly advantageous for this.

In FIGURE 2, a metal roller 13 rotating in the direction of motion of the image carrier is mounted at a small distance above the element 10 over which the carrier slides. In front of the roller in the direction of motion of the image carrier is a nozzle 14 for spraying the pure dispersion medium, the spray being incident both on the image just developed and on the roller 13. The fluid flowing to the sides and over the ends of the sheet is collected in the funnel 15 and fed back to the spraying nozzle 14 by way of a pump and a storage container, not shown.

When the apparatus described above is used for reversal development, very good images with strong contrast are obtained which can be used almost immediately after leaving the apparatus. The images have only a slight background tint if they are not rinsed after developing. There is none at all if they are rinsed, after developing, with an auxiliary device such as that shown in FIGURE 2.

It will be obvious to those skilled in the art that many modifications may be made within the scope of the present invention without departing from the spirit thereof, and the invention includes all such modifications.

What is claimed is:

l. An apparatus for the development of latent electrostatic images with a dispersion developer which comprises an application roller, wiper means in engagement with the roller and defining a trough-shaped reservoir between itself and the periphery of an upper portion of the roller, means for maintaining a supply of developer liquid in the reservoir, and means for passing an image carrier downwardly past a lower portion of the application roller on a side of the latter opposite the wiper means whereby the image carrier is held lightly against the application roller while the latter rotates in the same direction and at substantially the same speed of movement as the image carrier, the developer liquid being carried by the roller upwardly from the reservoir and over the top of the roller to pass downwardly onto the image carrier, and a film of developer liquid being maintained between the surface of the application roller and the image carrier.

2. An apparatus according to claim 1 in which the means for passing an image carrier past the application roller comprises a pair of feed rolls and guide means to hold the image carrier against the application roller.

3. An apparatus according to claim 2 in which the guide means has two spaced guide edges parallel to the axis of the application roller.

4. An apparatus according to claim 1 including a pair of squeeze rolls through which the image carrier is passed after development.

5. An apparatus according to claim 1 in which the means for maintaining a supply of developer liquid in the reservoir includes nozzle means.

6. An apparatus according to claim 1 including means for rinsing the image carrier after development.

7. An apparatus according to claim 6 in which the latter means includes a roller, adapted to contact the surface of the image carrier, and spray nozzle means.

8. An apparatus according to claim 2 in which the application roller.

References Cited UNITED STATES PATENTS Pomper 118262 X Armstrong 1187 X Schornstheimer.

Magnusson 118637 York 118-637 Keyworth et al. 118259 X Ostensen 118637 Marx et al. 118637 Fukuda 118-259 X CHARLES A. WILLMUTH, Primary Examiner.

guide means has one guide edge parallel to the axis of the 15 P. FELDMAN, Assistant Examiner. 

